1. Field of the Invention
The present invention relates to an operational amplifier and more particularly to an operational amplifier capable of preventing, without restricting an input signal voltage range, an output terminal from becoming inconstant when an input terminal is open circuited.
2. Background Art
FIG. 3 is a circuit diagram of a conventional operational amplifier. The conventional operational amplifier is comprised of comparators 301 and 303, reference voltage circuits 302 and 304, an OR circuit 305, a constant current circuit 306, an operational amplifier 106, resistors 307 and 308, input terminals 101 and 102, an output terminal 103, a supply terminal 104, and a ground terminal 105.
The operation of the conventional operational amplifier will be described.
The comparator 301 outputs a high signal when the voltage of a non-inverting input terminal exceeds the voltage of the reference voltage circuit 302. The voltage of the reference voltage circuit 302 is set to a voltage that is lower than a voltage VDD of the supply terminal 104. When the input terminal 101 is placed in an open state, the non-inverting input terminal of the comparator 301 is pulled up to the voltage VDD by the resistor 307. This enables the comparator 301 to detect that the input terminal 101 has been open circuited.
The comparator 303 outputs the High signal when the voltage of the inverting input terminal reduces to be lower than the voltage of the reference voltage circuit 304. The voltage of the reference voltage circuit 304 is set to a voltage that is higher than a voltage VSS of the ground terminal 105. If the input terminal 102 is open circuited, then the inverting input terminal of the comparator 303 is pulled down to the voltage VSS by the resistor 308. This enables the comparator 303 to detect that the input terminal 102 has been open circuited.
If an output signal of either one of the comparator 301 and the comparator 303 becomes a high signal, then the OR circuit 305 outputs a high signal. The constant current circuit 306 stops, so that the output terminal 103 of the operational amplifier 106 exhibits high impedance, making it possible to externally notify that the input terminal of the operational amplifier has been open circuited (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-143239
However, according to the conventional operational amplifier, there has been a problem in that an input terminal resistance cannot be increased in a normal operation mode, thus inconveniently restricting the voltage range of input signals that can be handled by the operational amplifier.
There has been another problem in that the output of the output terminal becomes inconstant when the input terminal is open circuited, resulting in an unstable operation of a circuit connected to the output terminal of the operational amplifier.